The present invention relates to an ion-trap mass analyzing apparatus in which an RF electric field is generated in an inter-electrode space to once stably capture all ion species contained in a sample, resonate target ions as a subject of mass separation and emit the target ions from the inter-electrode space to thereby perform mass separation.
In a conventional ion-trap mass analyzing apparatus, an electric field is generated symmetrically on ion inlet and outlet sides in order to keep z-direction oscillation of ions uniform.
For example, in U.S. Pat. No. 5,693,941, two end cap electrodes are disposed so as to be asymmetrical with respect to the central point of a ring electrode but a voltage applied between the two end cap electrodes is adjusted to generate an electric field in an inter-electrode space symmetrically on the ion inlet and outlet sides. Because the voltages themselves applied to the two end cap electrodes are made asymmetrical in accordance with the positional asymmetry of the two end cap electrodes, the internal electric field becomes symmetrical. As a result, the number of ions passing through an aperture in the end cap electrode on the side where a detector is disposed is increased without change in the behavior of ions compared with a conventional symmetrical ion trap to thereby attain improvement of sensitivity.
The conventional ion-trap mass analyzing apparatus has a problem as follows. That is, a mass shift phenomenon that the position of a mass peak is displaced from a position indicating a correct ion mass number may occur.